There are two main types of methods for producing fresh water from salt water.
The most widely used methods are based on a principle of reverse osmosis or ultrafiltration. These methods consist in filtering the salt water through semi-permeable membranes which allow the water to pass through but stop the ions (primarily sodium, potassium and chlorine). By applying pressure higher than the osmotic pressure, the water passes through the semipermeable membrane, thus enabling “filtration” of the ions and the obtainment of desalinated water. These methods are very energy-consuming, and, furthermore, the ultrafiltration membranes used are themselves very costly and have limited service lives, which further increases the cost of the fresh water produced. In addition, it is often necessary to use chelating agents prior to filtration in order to trap the ions likely to contaminate the membranes. In addition to increasing the cost per liter of water produced, these chelating agents, which are harmful to the environment, end up in the brine. This entails additional costs for retreating the brine and, depending on circumstances, pollution of the environment, adding to the already significant pollution caused by the brine.
In addition to these ultrafiltration and osmosis methods, distillation methods exist, which consist in producing vapour by bringing liquid water to the boiling point, and in then condensing the water vapour in order to obtain pure water.
These distillation methods are generally very costly in terms of heat energy. This energy is most often supplied in the form of electricity, by means of resistors, most often produced from fossil, nuclear or hydroelectric fuels. The amount of energy required to produce one m3 of water using these methods is very significant, which, in addition to a high production cost, produces pollution and requires the required energy to be available locally.
Alongside conventional heat distillation, solar heat distillation is sometimes used. This method consists in using solar energy to produce the water vapour that will be condensed. Although the latter method uses free energy to produce water vapour, the surface area required to produce one m3 per day is very significant: more than 100 m2. Furthermore, the required production equipment is difficult to move about.
Numerous wastewater purification methods are similar to desalination methods using filtration, with even more significant constraints on the poisons in the membranes and filters, which are found in even more significant amounts in wastewater. However, the majority of these methods aim to sufficiently purify the water prior to discharging same into the natural environment and not to produce water that is sufficiently pure for household or drinking purposes. In addition, in wastewater purification methods, treatment of the biodegradable organic compounds most frequently involves methods consisting in the aerobic degradation of the organic compounds, with a release of CO2; this involves the loss of the energy potential of said compounds. Wastewater methanisation methods which are compatible with fresh water recycling are not very widespread and are very difficult to implement on a small scale.
Finally, in the production of fresh water, very few methods take account of the potential represented by the water vapours as well as the volatile organic compounds present in or released into the atmosphere by household human activities (food cooking, showering, aerosol container gases . . . ). For example, human intestinal gases or gases resulting from animal husbandry account for a significant portion of the methane present in the atmosphere. It is interesting to note that water vapour and methane represent the two greenhouse effect gases, and therefore the impact thereof is the most significant, well ahead of CO2.
Capturing these gases in order to convert same into liquid water and CO2, for example, in addition to producing liquid fresh water, which is essential for many regions, would enable involvement in slowing down the increase in the primary greenhouse effect gases in the atmosphere and to therefore control global warming.
Numerous household air exhaust systems (toilet bowl exhaust, CMV exhaust, range hood exhaust, agricultural building exhaust . . . ) enable household gases to be exhausted (water vapour, methane . . . ) from the buildings. On the other hand, few systems enable a recycling and upgrading of these gases which is compatible with the household environment.
The method and equipment described in the remainder of this document enable sterile, fresh water (without any microorganisms) to be produced from salt water or dirty water, accompanied by the atmospheric water vapours and volatile organic compounds present in the atmosphere self-sufficiently in terms of energy.